The U.S. appears to have had little trouble convincing its allies that the September 11 terror attacks are the handiwork of Osama Bin Laden's Al Qaida network. And it's becoming easier to see why: TIME has learned that U.S. authorities have evidence that can place the hijackers' suspected ringleader, Mohamed Atta, along with several others in his group, at al-Qaida camps in Afghanistan.

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Intelligence sources have also told TIME that other evidence suggests that Atta and several others in the group met with senior Al Qaida leaders, most notably Ayman al-Zawahiri. The Egyptian Islamic Jihad leader is believed to be Bin Laden's deputy, and the top operational commander of Al Qaida's networks.

Britain on Thursday released a document setting out the basis for a case against Al Qaida for the September 11 attack, which indicates, among other things, that some of the hijackers had met with Al Qaida officials, and that Bin Laden had warned in the days before the attack that he was about to launch a major attack on America. The British document, based on telephone intercepts and information gleaned from interrogations in Europe since the attacks in New York and Washington, also reveals that orders were sent to a number of the network's key operatives to return to Afghanistan by September 10.

Al Qaida's often diffuse structures are designed specifically to avoid leaving behind trails of command responsibility detectable to investigators probing specific attacks. Proving that Atta and others had spent time at Al Qaida camps in Afghanistan and had met with al-Zawahiri would therefore constitute a major breakthrough in the investigation.